Powerful Millennial-Scale Solar Luminosity Cycles and Their Influence Over Past Climates and Geomagnetic Field

D. A. Stoykova, Y.Y.Shopov, D.Ford+, L.N.Georgiev, L.Tsankov. (Faculty of Physics, University of Sofia, James Bouchier 5, Sofia 1164, Bulgaria; ph. ++359(2)625 68 36; Fax: ++359(2)962 52 76; E- Mail: YYShopov@Phys.Uni-Sofia.BG); +(Geography Dept., McMaster Univ., Hamilton, Ontario, L8S 1K4, Canada; ph. ++(905) 546 45 37; Fax: ++(905) 546 04 63; E-mail: dford@mcmail.CIS.McMaster.CA)

Calcite speleothems luminescence depends exponentially upon soil temperatures that are determined primarily by solar visible and infrared radiation. So microzonality of luminescence of speleothems was used as an indirect Solar Insolation (SI) proxy index. For Cold Water cave, Iowa, US we obtained high correlation coefficient of 0.9 between a luminescence record and the experimentally observed Solar Luminosity Sunspot index.

We measured a luminescent speleothem record from Jewel Cave, South Dakota, US. It is still the only available experimental solar insolation proxy record with sufficiently long duration to reproduce the orbital variations. This record covers 89300- 138550 years B.P. with high resolution (34 years) and precision of measurements better than 1%. It reveals determination of millennial and century cycles in the record. The record has been dated by 6 TIMS U/Th dates with 2 sigma error ranging from 0.8 (for 89.3 kyrs) to 5.5 kyrs (for 138.5 kyrs).

This solar insolation proxy record contains not only orbital variations, but also solar luminosity self variations, producing many cycles with duration from several centuries to 11500 years. The most powerful non- orbital cycle is 11500 years cycle (as powerful as the 23000

a. orbital cycle in our record). It was found previously to be the most

intensive cycle in the delta C-14 calibration record and was interpreted

to be of geomagnetic origin. Our recent studies suggest, that this is a

solar cycle modulating the geomagnetic field. We found also cycles with duration of 6200, 4400, 3400, 2500, 2300, 1900 and 1460, years (in order of decreasing intensity) with amplitude ranging respectively from 16 to 4 % of the amplitude of the main orbital cycle presented in our record.

Latest results suggest that this millennial solar luminosity cycles can produce climatic variations with intensity comparable to that of the orbital variations. Known decadal and even century solar cycles have negligible intensity (about 100 times less intensive) relatively to these cycles. Solar luminosity and orbital variations both cause variations of solar insolation affecting the climate by the same mechanism. In spite their influence over the geomagnetic field involve fundamentally different mechanisms, determined by the properties of the solar wind.